1. Technical Field
The present invention relates to electro-optic crystal modulated laser systems and, more particularly, it concerns correction of the electro-optic crystal birefringence within the laser system.
2. Discussion
Conventional crystal modulated lasers, such as waveguide CO.sub.2 lasers, have an electro-optic crystal positioned within the laser's resonant cavity to frequency modulate the laser light produced by the laser gain medium. This is accomplished by applying a periodically changing voltage to the crystal. The periodically changing voltage causes the index of refraction of the crystal to change in synchronism with the modulation voltage. This changes the effective optical length of the laser resonator, causing the frequency of the laser light to be modulated.
The linear FM chirp laser, as disclosed in U.S. Pat. No. 4,666,295, which is incorporated by reference herein, is an example of a crystal modulated laser which produces a frequency-chirped output from the CO.sub.2 waveguide laser by applying voltage to an intra-cavity FM-cut CdTe electro-optic modulator. The applied voltage changes the refraction index of the crystal which, in turn, produces the FM chirp. This linear FM chirp laser system is deployed within a laser radar system to provide the system with the needed tuning range and linearity for pulse compression.
In accordance with laser principles, the amplitude of the output modulated light or the FM chirp, should change little if the overall frequency excursion is considerably smaller than the gain bandwidth. However, contrary to this principle, the frequency-chirped output produced by the linear FM chirp laser systems have been showing large dips in amplitude in the frequency spectrum. This phenomenon becomes more apparent in the higher gain laser systems.
This laser amplitude variation or AM dip is the result of birefringence in the electro-optic crystal. Birefringence is a property of certain crystals characterized by a different index of refraction for different light polarizations. A highly birefringent crystal can rotate light from one polarization to a different polarization, producing a loss which reduces the laser efficiency. In the case of the linear FM chirp laser, a portion of the linearly polarized laser light within the resonant cavity becomes elliptically polarized upon passage through the FM-cut CdTe crystal. Since the elements within the resonant cavity are linearly polarization-sensitive, the elliptical polarization caused by the birefringence subjects the modulated output to an amplitude loss.
It is found in practice that these large amplitude variations have caused a number of problems in the laser radar systems. For example, the variations make locking the laser frequency extremely difficult. The lockloop within the system relies upon small amplitude changes caused by gain variations to hold the lazing frequency at line center. When the large amplitude variations caused by the crystal birefringence swamp-out the small amplitude changes relied upon by the lockloop, frequency control becomes difficult.
There is no known practical method for correcting the amplitude variation in the modulated output. One method currently being used is to operate the laser while maintaining the crystal at a specific temperature. Since the birefringence within the crystal is generally unknown prior to laser operation, the procedure must be performed experimentally to determine the best working temperature. Through an exhaust of trial and error process, a very narrow temperature range, usually within a half degree, can be found in which the laser will operate with minimal amplitude fluctuation. Because this procedure is extremely time consuming and requires expensive electronic temperature feedback circuits to maintain the temperature within the required range, much of the amplitude variation goes uncorrected.
There is, therefore, a real need to provide an effective method and apparatus for correcting the birefringent induced losses within an electro-optic modulated laser system.